linux-kernel - Re: [PATCH] x86: add hintable NOPs emulation

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Message-ID: <20250821124804.GP3289052@noisy.programming.kicks-ass.net>
Date: Thu, 21 Aug 2025 14:48:04 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Marcos Del Sol Vives <marcos@...a.pet>
Cc: linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H. Peter Anvin" <hpa@...or.com>, Brian Gerst <brgerst@...il.com>,
	Uros Bizjak <ubizjak@...il.com>, Ard Biesheuvel <ardb@...nel.org>,
	David Kaplan <david.kaplan@....com>,
	"Ahmed S. Darwish" <darwi@...utronix.de>,
	Kees Cook <kees@...nel.org>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>, "Xin Li (Intel)" <xin@...or.com>,
	Sabyrzhan Tasbolatov <snovitoll@...il.com>
Subject: Re: [PATCH] x86: add hintable NOPs emulation

On Wed, Aug 20, 2025 at 03:34:46AM +0200, Marcos Del Sol Vives wrote:
> +static bool handle_hnop(struct pt_regs *regs)
> +{
> +	struct thread_struct *t = &current->thread;
> +	unsigned char buf[MAX_INSN_SIZE];
> +	unsigned long nr_copied;
> +	struct insn insn;
> +
> +	nr_copied = insn_fetch_from_user(regs, buf);
> +	if (nr_copied <= 0)
> +		return false;
> +
> +	if (!insn_decode_from_regs(&insn, regs, buf, nr_copied))
> +		return false;
> +
> +	/* Hintable NOPs cover 0F 18 to 0F 1F */
> +	if (insn.opcode.bytes[0] != 0x0F ||
> +		insn.opcode.bytes[1] < 0x18 || insn.opcode.bytes[1] > 0x1F)
> +		return false;

FWIW, you need to check for insn.opcode.nbytes == 2.

> +	if (!t->hnop_warn) {
> +		pr_warn_ratelimited("%s[%d] emulating hintable NOP, ip:%lx\n",
> +		       current->comm, task_pid_nr(current), regs->ip);
> +		t->hnop_warn = 1;
> +	}
> +
> +	regs->ip += insn.length;
> +	return true;
> +}